1. Field of the Invention
The present invention relates to a producing method of a mask in which an original or master plate to be transferred onto a substrate such as a wafer in a lithography process for producing semiconductor devices, image pickup devices (CCDs etc.), liquid crystal displays, plasma displays, thin film magnetic heads and the like. The present invention also relates to an exposure method and an exposure apparatus used for this producing method. For example, the invention is suitably used for producing a mask and the like such as a transparent reticle using excimer laser as exposure beam, a reflective reticle using EUV light such as soft X-ray as the exposure light, and a membrane structure using electron beam as the exposure beam.
2. Description of the Related Art
Conventionally, when a semiconductor device is produced, in order to transfer a pattern of a reticle (or photo-mask) as a mask onto each shot region of a wafer on which a photoresist is applied, an i line (wavelength 365 nm) of a mercury lamp as exposure beam, or KrF excimer laser (wavelength 248 nm) is used, and a projection exposure apparatus (stepper or the like) using a projection optical system having the number of openings NA of about 0.7 is used. If a wavelength of the exposure beam is defined as λ and a predetermined process coefficient is defined as k, resolution on a wafer is expressed as. k×λ/NA. Therefore, a conventional minimum line width of an image of a line and space pattern that can be transferred onto the wafer is about 180 nm. The size of the conventional reticle is usually 5×5 inches or 6×6.
In this case, since a projection magnification of a projection optical system is about ¼ or ⅕, a line width of a pattern of a reticle corresponding to the minimum line width (when the projection magnification is ¼) is about 720 nm. A conventional reticle having such a pattern is produced by directly forming the original pattern on a predetermined substrate using an electron beam drawing apparatus.
As described above, the conventional reticle is produced by directly forming, onto a substrate of about 6×6 inches at the maximum, an original pattern whose a minimum line width becomes about 180 nm on a wafer. However, since the electron beam drawing apparatus continuously forms various portions of the original pattern with beams of a predetermined cross sectional shape, there is inconvenience that the pattern-forming time becomes long and the producing time of reticle becomes long. Especially, since the same reticles are usually used as working reticles concurrently by a plurality of producing lines, it is necessary to produce a plurality of reticles having the same original pattern. At that time, the pattern of each of the working reticles is formed by the electron beam drawing apparatus, time required for producing the reticles becomes extremely long.
Further, precision of about 5% of the minimum line width in an entire surface of the reticle is required as pattern-forming precision. Therefore, if the minimum line width is 720 nm, precision of about 36 nm is required. Thus, when the size of the reticle is 6×6 inches, precision of about 36 nm (˜2.4×10−7) is required for length of about 1.50 mm. Such precision is almost limit of the current electron beam drawing apparatus when drift of electron beam is taken into consideration.
Further, the resolution will further be improved so as to meet the increased packing density of the semiconductor device and the like. That is, for future several years, in order to transfer a pattern having the minimum line width of about 180 to 100 nm onto a wafer, ArF excimer laser light (wavelength is 193 nm), F2 laser light (wavelength is 157 nm) and laser light of vacuum ultraviolet (VUV) such as solid laser and the like are under review. As a reticle for exposure beam of the vacuum ultraviolet longer than about 100 nm, a transparent reticle using fluorite (CaF2) as a substrate can be used.
In order to further enhance the resolution for the next generation semiconductor device, an exposure apparatus in which extreme ultraviolet light (EUV light) such as soft X-ray (wavelength is about 13 to 6 nm) is used as exposure beam, and reflection system of reduced magnification using a combination of about three to five concave mirrors and convex mirrors is used as the projection optical system is under development. When the EUV light is used, since there is not optical material having excellent transmittancy, it is considered that a reticle to be used is a transparent reticle.
The use of an electron beam exposure apparatus in which a mask (stencil mask or the like) of a membrane structure having predetermined opening patterns in thin film members formed on a wafer into a lattice shape is irradiated with electron beam, an image of the opening pattern in the film member is transferred onto a substrate to be exposed while stitching screens at reduced magnification, thereby transferring a pattern of large area at high resolution is also under review. It is expected that resolution of about 130 to 30 nm can be obtained using the exposure apparatus or the electron beam exposure apparatus using the EUV light.
In order to obtain resolution of about 180 to 30 nm on a wafer, if the projection magnification of the projection optical system is ¼, the minimum line width of the reticle pattern is about 720 to 120 nm. It is expected that the size of the future reticle will be about 9×9 inches. Therefore, if the pattern-forming precision is about 5% of the minimum line width, precision required for the electron beam drawing apparatus is about 36 to 6 nm (about 1.6×10−7 to 2.6×10−8) with respect to a length of about 230 nm, but it is difficult under present circumstances to realize such a high precision. Further, if the area of the reticle becomes greater and the pattern becomes finer, the pattern-forming time becomes longer. Therefore, especially when a plurality of working reticles are produced, there is inconvenience that the producing time becomes excessively long.
In recent years, attention is directed toward technique for disposing previously designed various circuit units such as CPU or memory into a predetermined arrangement, these units are connected to one another through wires, thereby producing a semiconductor device that can achieve a desired function as in a case in which ASIC (application-specific IC) is produced. According to this technique, it is possible to develop semiconductor devices having various functions in a short time and thus, it is expected that the technique will widely be utilized in fields of multimedia, digital TV and the like. However, in such a case also, if the original pattern of each reticle is formed using the electron beam drawing apparatus, since the producing time of the reticle becomes long, there is inconvenience that developing time can not be shortened so much especially when various semiconductor devices are developed.
Thereupon, recently, a method in which an original pattern having an enlarged pattern on a reticle is prepared, this original pattern is divided into a plurality of parent patterns, they are formed on master reticles, images of the patterns of the plurality of master reticles are transferred onto a glass substrate while stitching screens using reduction projection type exposure apparatus, thereby producing reticles (working reticles) for actually light exposure is under review. When the image is transferred while stitching screens in this manner, it is necessary to reduce stitching error of a boundary portion (stitching portion) of adjacent parent patterns, and to reduce variation in exposure light amount in the vicinity of the boundary portion.
As an exposure method that can be used to reduce the stitching error and to reduce variation in exposure light amount, there is a method as disclosed in Japanese Patent Application Laid-open No. 6-132195 and corresponding U.S. Pat. No. 5,477,304 in which in order to transfer an image of a reticle pattern in each shot region on a wafer while stitching the screens, illumination distribution of illumination region of the exposure light is formed into a trapezoidal shape in which opposite ends are gradually lowered, and image of adjacent reticle patterns are overlapped on a boundary portion of a predetermined width. As a first method for forming the illumination distribution of the illumination region into the trapezoidal shape, there is a method in which a disposing surface of a reticle blind (variable field aperture) for defining the illumination region is defocus on the illumination region (pattern surface of reticle). According to this method, however, when shape of the opening aperture of an illumination optical system is switched from a circle (normal illumination) to a plurality of decentered opening (deformed illumination), there is an adverse possibility that the shape of the illumination distribution is not trapezoidal shape.
In order to prevent the shape of the illumination distribution from being deformed, the defocus amount of the reticle blind may be varied in accordance with illumination condition for example, but there is inconvenience that the mechanism of the illumination optical system is complicated.
Further, in order to form the illumination distribution of the illumination region into substantially the trapezoidal shape, there is proposed a method for moving a blade constituting the reticle blind into exposure light. However, there is inconvenience that this method also complicates a driving mechanism of the reticle blind and the mechanism of the illumination optical system is complicated.
There is considered a method for forming the illumination distribution of the illumination region into the trapezoidal shape by making ends of the reticle blind disposed in conjugate position with the illumination region semi-transparent. According to this method, however, if a foreign substance is attached to the semi-transparent portion, uneven illumination is generated in the illumination region. In order to avoid this, it is necessary to enhance the precision of a dustproof mechanism for gas supplied to the illumination optical system. Therefore, there is inconvenience that the mechanism of the illumination optical system is complicated.